1 Introduction

Solving the global energy problems of modern society requires the development of various areas of “green technology”. Today, in the field of semiconductor photocatalysis, TiO2 is an undoubted leader both in terms of demand and research intensity (including the synthesis and testing of a wide range of new composite titania-based materials) due to the unique combination of its properties and, first of all, non-toxicity, chemical stability and low cost [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. The efficiency of TiO2 photocatalyst is determined by numerous parameters: crystallinity, phase composition, crystallite size, particle morphology, porosity, developed surface area, surface organization, etc. [16, 20,21,22,23,24]. The classic definition of “the ideal powder should be pure, stoichiometric, dense, spheroidal, and nearly monodisperse” was made by Brinker and Scherer back in 1990 [25]. 5 years later, Antonelli and Ying [26] proposed their own method of the sol–gel synthesis of mesoporous TiO2 with a high specific surface, thereby indicating the main direction of researchers’ efforts to improve TiO2 photocatalysts.

Undoubtedly, the sol–gel template method still remains one of the most advanced methods for producing mesoporous oxides. Its combination with widely used solvo (hydro) thermal processing of an intermediate amorphous organo-inorganic product allows controlling the processes of formation of unique TiO2 materials with a certain morphology and texture at the nanoscale level [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41].

Hydrothermal treatment at relatively low temperatures (< 200 °C) gives the possibility to obtain highly homogeneous nanocrystalline TiO2 powders [16, 29, 31, 35, 37, 42, 43]. However, in the process of manufacturing a highly efficient photocatalyst, the obtained powders, as a rule, require calcination at temperatures not lower than 500 °C to achieve full catalytic potential [23, 24, 29, 36, 40, 44,45,46,47].

TiO2 exists mainly in three different crystalline forms: anatase, rutile and brookite. Anatase has the highest photoactivity [48]. However, the interest in obtaining TiO2 nanocomposites, which consist of two or three polymorphs (relatively few works for latter), is not diminishing. The results obtained in these works sometimes lead to contradictory conclusions when comparing of the photocatalytic activity of the TiO2 mixed-phase in various redox processes. Many authors reported that the photocatalytic activity of mixtures of TiO2 phases in a number of processes is higher in comparison with the pure single polymorphs [14, 16, 20, 21, 24, 49,50,51]. It is attributed to the synergistic effect of different positions of the energy levels of different phases of titanium dioxide, which leads to the spatial separation of photogenerated charges in mixed-phase materials, decrease in the negative electron – hole recombination of charges, and increase in the activity of these materials in various redox processes [20, 31, 36, 52,53,54,55,56,57,58].

Earlier, under conditions of neutral hydrolysis, samples of thermally stable meso-nc-TiO2 with a pure anatase structure were obtained [59,60,61,62,63], demonstrating different textures with well-defined spherical morphology (micro- and nanospheres) and a narrow pore distribution. Calcined at 500 °C after hydrothermal treatment at 175 °C TiO2 samples had a radically more developed porous structure compared to samples without hydrothermal treatment [59]. Mesoporous hierarchical TiO2 microspheres ranging in size from 0.6 to 3.0 μm, were formed by aggregation and agglomeration of homogeneous nanoparticles (30–70 nm), in turn formed by primary spherical particles—anatase crystallites of about 10 nm (XRD). The introduction of small amounts of surfactant and a lanthanum salt into the reaction mixture led to increase in crystallinity, change in morphology, and development of the of meso-nc-TiO2 texture. Spherical particles of micrometer scale in the presence of La3+ ions are not formed. It should be emphasized that anatase is the only crystalline phase (in the absence of signs of phase transformation) of meso-nc-TiO2 formed under neutral conditions. Almost all the obtained samples are highly effective photocatalysts for water reduction, gas-phase oxidation of alcohol and benzene [61,62,63,64].

In the present work, the above approach was used for synthesis of meso-nc-TiO2 samples of both anatase and various phase compositions (anatase, brookite, rutile), as a combination of two or three phases, using hydrochloric acid (HC1) as a catalyst for hydrolysis of the TiO2 precursor. All samples were calcined at 500 оС.

2 Materials and methods

2.1 Materials

Titanium tetrabutoxide (IV), dibenzo-18-crown-6 (Sigma-Aldrich), dodecyldi- methylethylammonium bromide (Fluka), НCl acid, n-butanol, NaCl and ethanol (96%) (Himlaborreactiv) were used. Commercial TiO2 Evonik-P25 (Evonik Corp) was chosen as the standard photocatalyst.

2.2 Synthesis

Samples of meso-nc-TiO2 with different phase compositions were obtained using the modified sol–gel method described in detail in our works [59, 60]. Dibenzo-18-crown-6 (in the form of the sodium complex [Na(DB18C6)]Cl due to the low solubility of crown ether in butanol (BuOH)) was used as a template in the presence (or in the absence) small additives of the cationic surfactant dodecyldimethylethylammonium bromide (DDMEABr) and/or lanthanum salt. The calculated amounts of reagents were sequentially dissolved in butanol (BuOH). Titanium tetrabutoxide (TBOT) was added dropwise under vigorous stirring. In contrast to the mentioned works, in this work, TBOT was hydrolyzed in an acidic medium using hydrochloric acid as catalyst.

The reaction mixture was left under a glass hood in air at room temperature (without stirring) until gel formation has stopped. HTT was carried out at 175 °C for 24 h. All samples (both treated and untreated hydrothermally) were calcined at 500 °C for 4 h.

Samples obtained under various conditions were designated as Tn and TnH, where n is the number of the sample and H denotes hydrothermal treatment. The molar ratios of the reaction mixture used for the synthesis are shown in Table 1:

Table 1 Molar ratios of starting reagents, used for meso-nc-TiO2 synthesis
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2.3 2.3 Characterization

A.Dron-3.M diffractometer with Cu Kα (λ = 1.5406 Ȧ) radiation was used for the X-ray diffraction (XRD) analysis to determine the crystal structure and crystallinity of TiO2. The average size of TiO2 crystallites (2R) was estimated from the broadening of the anatase peak at 2Θ = 25.4o (101) in XRD-spectra using well-known Scherrer’s equation.

The morphology of the samples was observed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) оn a JEOL JEM 1230 and а JEOL JSM 6060 LA microscopes respectively. Energy dispersive X-ray (EDX) analysis were performed with a Tescan Measure 3 microscope, equipped with an Oxford X-max energy-dispersive X-ray assay for 80 mm2, for an accelerating voltage of 5–20 kV. Electron diffraction patterns were obtained on a transmission electron microscope Selmi TEM-125К with the accelerating voltage of 100 kV.

In some cases, the samples were pre-treated with an ultra sound disperser UZDN-A (radiation power 130 W) for 5 min.

N2 adsorption–desorption isotherms were recorded at 77 K using an Autosorb-6 automated gas adsorption analyzer (Quantachrome). Prior to the measurements, samples were degassed at 473 K for 12 h. The specific surface area (SBET) of the samples was determined by the BET method. The average pore size (Dp) was calculated using BJH method. The NLDFT method (cylindrical pore model) was used to calculate the pore size distribution from the desorption branch. The mesopore volume (Vp) was determined using amount of N2 adsorbed at relative pressure p/p0 of 0.997.

2.4 Photocatalytic oxidation studies

The photocatalytic activity of the obtained meso-nc-TiO2 samples was studied in the gas-phase reaction of ethanol oxidation. A 50 mg titanium dioxide sample pressed onto a 1 × 3 cm copper support was put into a 150 cm3 glass reactor, equipped with a magnetic stirrer and membrane for sampling. The required amount of alcohol was introduced into the reactor using a microsyringe. For complete evaporation of the alcohol and adsorption equilibrium establishing, the reactor was kept without irradiation with stirring of the gas mixture for ~ 2 h. The samples were irradiated with the focused light of a DRSh-1000 mercury lamp. A region with λ = 310–390 nm was isolated from the radiation spectrum by means of filters. The light intensity was measured using a ferrioxalate actinometer (I = 5 µmol of quants per min). The concentration of the starting substrate was determined using a CHROM 5 gas chromatograph equipped with a flame ionization detector and a column packed with Porapak Q. The initial oxidation rates were determined using the initial linear region over approximately 30 min of irradiation as the ratio of the change in ethanol concentration in the reactor to the irradiation time.

3 Results and discussion

Samples of meso-nc-TiO2 with different phase composition were obtained by a simple modification of the previously proposed approach [59] through changing the conditions of TBOT hydrolysis, namely, the transition from a neutral to an acidic environment using a well-known acid catalyst—hydrochloric acid [23, 53, 65]. HCl is the most commonly used due to the lower electronegativity of the chloride anion compared with others and bonding of the Cl- ion to the Ti atom. HCl promotes the hydrolysis reaction versus the condensation reaction [35] as well as serves as an electrolyte to prevent particle growth or agglomeration through electrostatic repulsion. The main concept of such optimization of the previous approach [59, 60] was to combine the factors: HTT, small additives of surfactant and (or) lanthanum salt, the influence of reagents concentration in the presence of HCl on the phase composition of meso-nc-TiO2 materials (at the same calcination temperature—500 оС).

Figure 1 shows the diffraction patterns of meso-nc-TiO2 samples obtained in an acidic medium in the presence of HCl. The phase content of each polymorph was determined by the main diffraction peaks of anatase, rutile and brookite, designated as (101), (110) and (121) reflexes, respectively, using the well-known calculation method [66]. The diffraction patterns of samples T2 and T4 contain weak narrow peaks at 32° and 45.5° (2Θ), which can be attributed to NaCl, occluded by the main product during the synthesis [60]. The content of NaCl in the samples was confirmed by the EDX (Table S1). Calculated from XRD date, size of anatase crystallites in samples under consideration is about 10 nm (see Table 2).

Fig. 1
figure 1

XRD patterns of meso-nc-TiO2 samples, obtained in various synthetic conditions. Anatase (JCPDS-PDF card no. 21-1272), rutile (JCPDS-PDF card no. 21-1276), brookite (JCPDS-PDF card no. 29-1360)

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Figures 2 and S1 show the N2 adsorption-desorption isotherms and pore size distributions for meso-nc-TiO2 samples. All isotherms belong to type IV IUPAC classification with H1 and H2 hysteresis loops located at relative pressure p/po values higher than 0.6, indicating the formation of mesopores in the obtained samples [67, 68].

Fig. 2
figure 2

N2-adsorption/desorption isotherms and pore size distribution obtained by NLDFT of Т1, Т1Н (a) and Т4, Т4Н samples (b)

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Fig. 3
figure 3

TEM of Т1 (a, b) and Т1Н (c, d), Т4 (e, f) and Т4Н (g, h)

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Table 2 shows the phase composition and texture characteristics of meso-nc-TiO2 samples. As can be seen from Figs. 1 and 2, S1 and Table 2, the phase composition and texture characteristics of meso-nc-TiO2 samples were influenced by DDMEABr surfactant additive, lanthanum salt, and concentration of reagents in the initial reaction mixture during synthesis, as well as the heat treatment regime (HTT before calcination).

The efficiency of crown ether templating activity was improved by addition of minute amounts of surfactant using previously described method [60]. The possible mechanism of the process can be explained by the destructuring effect of the surfactant on the solvation shell of the crown ether complex. “Naked” oxygen atoms from the crown ether can react with structure-forming fragmens with Ti to form TiO2 mesophase framework. As can be seen from Table 2, the addition of DDMEABr lead to the decrease of anatase and brookite content and increase in the content of rutile, respectively (samples T1 and T2). The differences become more visible in a case of HTT processing of these samples (samples T1H and T2H). The presence of a surfactant additive opens possibility for the synthesis of T2H sample with the highest rutile phase content of 70%. The presence of a surfactant lead to a slight increase in the specific surface area (SBET) in the case of calcined samples T1 and T2 (39 m2/g and 48 m2/g), on the contrary, for the T1H and T2H samples (with preliminary hydrothermal treatment), there are almost twofold decrease of the corresponding SBET values (61 m2/g and 33 m2/g). However, surfactant did not affect the pore diameter of the specified pairs of samples (samples T1−T2 and T1H−T2H) in a great extent.

The addition of lanthanum salt promotes the increase the anatase content and decreases the brookite content regardless of the use of HTT (samples T1, T1H and T3, T3H), while the content of rutile decreases in the absence of HTT (samples T1 and T3) and increases at HTT (samples T1H and T3H). In the case of samples obtained with the simultaneous presence of DDMEABr and lanthanum salt (T1, T1H and T5, T5H, T6H), the three-fold increase of the anatase content is observed along with the a sharp drop of the brookite content and complete disappearance of the rutile phase. The phase composition of the sample undergoes especially profound changes when using HTT (samples T1H and T5H), when the three-phase composition turn into anatase monophase (Fig. 1; Table 2).

Addition of La3+ ions to the initial reaction mixture provided a significant increase in the specific surface area. So, for example, for samples T1 and T3, the value of SBET increases twofold (39 m2/g and 89 m2/g, respectively). Doping with small amounts of La3+ cations was successfully used for thermal stability increasing of mesoporous anatase [60]. The role of this cation in stabilizing the structure of mesoporous TiO2 has been explained by implantation of rare-earth cations into framework of the oxide system. Lanthanum is more electropositive than titanium and thus formation of the La–O bond causes partial transfer of electrons from the La–O bond to the Ti–O bond leading to strengthening of the latter. La3+ does not enter into the crystal lattice of TiO2 and was uniformly dispersed onto TiO2, supported by the results of EDX analysis (Table. S1). The temperature of anatase–rutile transformation significantly increases in the presence of La3+ as compared to pure TiO2.

Dilution of the initial reaction mixture in the presence of lanthanum salt (samples T3, T3H and T4, T4H, Table 1) lead to a twofold increase of the anatase content, along with the decrease of the content of brookite and rutile. For example, sample T3 comprises of anatase (55%), rutile (21%) and brookite (24%), and the corresponding sample T4 has only anatase (97%) and rutile (3%) phases. An increase of the concentration of reagents in the initial reaction mixture containing additives of DDMEABr and lanthanum salts (samples Т5Н, Т6Н and Т7Н) allows formation of one, two, and three phase TiO2 compositions (Fig. 1; Table 2). In the case of sample T7H (compared with samples T5H and T6H), additional amounts of Н2О and НСl, apparently, have a definite effect on the phase composition of this sample.

Upon dilution of the initial reaction mixture in 1.5 times, the specific surface area increases by a factor of ~ 1.5 (samples T3, T3H and T4, T4H), while the pore diameter decreases (samples T3H and T4H). The simultaneous introduction of surfactant additives and La3+ ions (samples T5 and T5H) lead to a tremendous increase in the specific surface area SBET of meso-nc-TiO2 samples compared to samples obtained only in the presence of surfactant (samples T2 and T2H) or La3+ ions (samples T3 and T3H).

As follows from a comparison of the texture characteristics of the samples presented in Table 2, the Т5Н sample has the largest specific surface area (SBET = 132 m2/g) and pore volume (Vp = 0.46 cm3/g). An increase of the concentration of the starting reagents (samples T5H and T6H) lead to a 1.5-fold decrease in the specific surface and a substantial increase in pore diameter (~ 25%) with a constant pore volume.

The values of the average sizes of anatase crystallites presented in Table 2 fall in the range of 7.0–10.4 nm, and the highest values are observed for samples T1 (9.8 nm) and T1H (10.4 nm) obtained without DDMEABr or lanthanum salt.

Table 2 Synthesis conditions of meso-nc-TiO2 samples, their specific surface area (SBET), pore volume (Vp), pore diameter (Dp), crystallite size and phase composition
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The choice of the heat treatment mode (calcination or calcination with preliminary hydrothermal treatment) significantly affects the phase composition of meso-nc-TiO2 nanocomposites (samples T1 and T1H, T2 and T2H, T3 and T3H, T4 and T4H, T5 and T5H) in all cases. For example, a T1 sample subjected to calcination only has a composition of anatase (29%), rutile (39%) and brookite (32%), and a T1H sample (preliminary hydrothermally treated)-anatase (34%), rutile (14%) and brookite (52%). It should be noted that among all meso-nc-TiO2 samples, the T1H sample (obtained in the absence of any additives) contains the highest amount of brookite.

Comparison of the texture characteristics of samples T2 and T2H (obtained in the presence of a surfactant), T3 and T3H, T4 and T4H (obtained in the presence of lanthanum salt), hydrothermal treatment before calcination lead to a decrease in the specific surface (~ 20–30%) and a significant increase in pore diameter (~ 20–80%) along with a change in pore volume (an increase of almost 1.5 times) only in the case of the T3H sample (Table 2). In the case of samples T5 and T5H (obtained with the simultaneous presence of La3+ ions and a surfactant), on the contrary, the T5H sample previously hydrothermally treated before calcination compared to a simply calcined sample T5 showed an increase in specific surface area (~ 35%) with a simultaneous increase in ~ 2.5 times the pore volume and in 2 times the pore diameter.

As follows from the TEM data (Figs. 3, S2–7), all samples of meso-nc-TiO2 has homogeneous, spherical primary particles (anatase phase). The sizes of anatase crystallites in the samples estimated from TEM are in good agreement with the corresponding values, calculated by the Scherrer formula (Table 2). Samples in which the percentage of the rutile and brookite phases are high (Т1, Т1Н, Т2, Т2Н, Т3, Т3Н, T7H), shows the presence of crystallites in the form of variously sized plates of hexagons and quadrangles shapes (Fig. 3a–d, Fig. S2–7). Electron diffraction patterns are in good agreement with the XRD (Fig. S8–9).

Figures 4, 5, 6 and 7, S10–18 show SEM images of meso-nc-TiO2 samples. As follows from Figs. 4, S10–12 there are no microspheres in samples of T1 and T1H, T2 and T2H obtained with and without HTT. Earlier [59,60,61, 63], we have showed that TiO2 samples obtained in a similar way under neutral conditions—microscopic particles with a well-defined spherical shape, were mesoporous TiO2 microspheres (1–3 μm), which consist of spherical homogeneous nanoparticles (secondary particles ) 30–50 nm in size, which in turn were formed by aggregation of anatase crystallites (primary particles) of about 10 nm in size. This led us to the conclusion that the presence of hydrochloric acid prevented the formation of microspheres. All meso-nc-TiO2 samples formed in acidic medium were aggregates of various sizes (from 100 to 500 nm). During ultrasonic treatment of samples T1H, T4, T4H, T5H, T6H, the aggregates break up into homogeneous nanoscale (40–100 nm) secondary particles of spherical or spheroidal shape (Figs. 4b, 6a, e, f, 7a, b, S15).

Fig. 4
figure 4

SEM of Т1 (a) and Т1Н (b-after ultrasonic treatment), Т2 (c) and Т2Н (d)

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Fig. 5
figure 5

SEM of Т3 (a, b) and Т3Н (c, d)

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Fig. 6
figure 6

SEM of Т4 (а-after ultrasonic treatment) and Т4Н (b), Т5 (c, d)

и Т5Н (e, f-after ultrasonic treatment)

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Fig. 7
figure 7

SEM of Т6Н (а, b-after ultrasonic treatment) and Т7Н (c, d)

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4 Photochemistry

For evaluation of photocatalytic activity of meso-nc-TiO2 samples, we used the gas-phase oxidation of ethanol as a model process for the oxidation of organic substrates [69,70,71,72,73].

Such an assessment is of particular interest because all titanium polymorph compositions were obtained within the same synthetic method using the same heat treatment mode (calcination at 500 °C) [74,75,76].

Histograms of the phase composition of meso-nc-TiO2 samples and the corresponding values of ethanol oxidation initial rates are shown in the Fig. 8a, b. As can be seen, all the samples significantly outperformed the commercial photocatalyst Evonik P25 in photocatalytic activity. The dependence of the photocatalytic activity of the obtained samples, except for T4 and T4H, in the process under study increases almost linearly with an increase in the content of the anatase phase (Fig. 9a). The activity of the T4 and T4H samples is significantly higher than the activity of the meso-nc-TiO2 samples with similar anatase content (T5, T5H, T6H) (Fig. 9a).

Fig. 8
figure 8

Phase composition (a) and the corresponding ethanol oxidation initial rates (b) of meso-nc-TiO2 samples

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Fig. 9
figure 9

The dependence of the ethanol oxidation initial rates in the presence of meso-nc-TiO2 samples upon anatase content (a) and SBET (b)

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Sample T4 has a composition of anatase (97%)-rutile (3%) phases, and sample T4H-anatase (85%)-rutile (4%)-brookite (11%) phases. The Т5Н sample (anatase ~ 100%), was noticeably inferior in activity to the Т4 and Т4Н samples. Samples T5 and Т6Н are very close in composition (88% anatase and 90% anatase, 12% brookite and 10% brookite, respectively), but, unlike the T4 biphase sample and the T4H three-phase sample, did not contain rutile phase.

Samples T4 and T4H also significantly deviate from the dependence of the photocatalytic activity of samples on their specific surface area (Fig. 9b). In this case, the activity of the T4 and T4H samples is significantly higher than that of the samples with a similar specific surface area (T5, T5H, T7H).

The observed picture can apparently be explained by the synergistic effect [16, 19, 77] due to the combination of the anatase and rutile phases in the phase compositions of T4 (anatase (97%)-rutile (3%)) and T4H (anatase (85%)-rutile (4%)-brookite (11%)). The authors of [77] have showed that even 2% content of rutile in anatase-rutile mixed-phase promotes the photocatalytic activity of TiO2. A significant increase in the photocatalytic activity of anatase composites with a small amount of rutile can be associated with the different positions of the energy levels in anatase and rutile [78], which leads to spatial separation of photogenerated charges between the components of the composite and a decrease in the unwanted electron-hole recombination (Fig. S19). In the case of composites of anatase with brookite, the synergistic effect is not observed, which is associated with a higher level of the conduction band of brookite compared to anatase [78] and the absence of separation of charges photogenerated in anatase between the components of the composite (Fig. S19). In other samples obtained containing rutile, the positive effect of charge separation is compensated by the relatively low content of the active phase of anatase.

5 Conclusions

Binary and ternary phase mixtures of mesoporous nanocrystalline titanium dioxide samples with different phase composition and texture were obtained by sol–gel synthesis (with or without hydrothermal treatment). The anatase phase, the content of which in the samples ranged from 16 to ~ 100%, consisted of homogeneous spherical anatase crystallites ~ 10 nm in size.

The phase composition of the meso-nc-TiO2 samples was formed by varying the concentrations of the reagents, using small additions of dodecyldimethylethylammonium bromide surfactant and/or lanthanum salt in the reaction mixture and hydrothermal treatment. All synthetized samples were calcined at the same temperature of 500 °C for 4 h. This is the crucial condition for formation a number of photocatalysts with different phase composition for a comparative assessment of their photocatalytic activity. Meso-nc-TiO2 samples with anatase content over 80% had the high level of photoactivity. The highest photocatalytic activity was demonstrated by the sample with an anatase phase (97%)-rutile (3%) composition.